1. Field of the Invention
The invention relates to processes for forming poly(vinylidene fluoride-co-trifluoroethylene) networked polymers. The networked polymers formed by the process have high electrostrictive coefficients.
2. Description of the Related Art
Electromechanically active materials convert electrical energy into mechanical displacements, with lower power consumption than electromagnetic motors. The electrically induced strain has at least two components including piezoelectric strain and electrostrictive strain, shown in Equation 1. While piezoelectric strains are proportional to the applied electric field, E, the electrostrictive strain increases quadratically with the field. Thus, materials with substantial electrostrictive coefficients offer the possibility of obtaining very large deformations while requiring low power consumption. Further, since the strain is proportional to the square of the field, the strain is in the same direction regardless of the polarity of the applied field. Applications of electrostrictive materials include sonar, sensors, transducers, actuators, and potentially robotics and artificial muscles.
s=dE+aE2xe2x80x83xe2x80x83(1)
Piezo-ceramics (e.g., lead zirconate titanate (PZT), lead zirconate, lead titanate, lead metaniobate, lead magnesium niobate) have been the material of choice for sensors and transducers, combining good electromechanical properties with a large modulus (xe2x89xa7100 GPa). Modulus is an important property of electromechanically active materials, since it contributes to the conversion efficiency, which is defined as the fraction of input electrical energy converted to mechanical energy. The drawbacks to ceramics include their brittleness (cyclic strains  less than 0.05%) and poor processability. Although ceramics are the dominant materials for such applications, there is great interest in developing electromechanically active polymers, given their superior processability and toughness, as well as lighter weight (especially important for aviation applications). Polymers offer potential advantages over ceramics, such as greater toughness, reduced weight (six times less dense than ceramics), and processability. Not only can polymers be formed into large, complex shapes, but they can also be used to produce small electromechanical devices by embossing a polymer film with microelectronic devices such as amplifiers.
Electromechanically active polymers must, of course, be polar. However, macroscopic alignment of molecular dipoles is limited for polymers, since they are rarely ordered on the microscopic level. Even among highly crystalline polymers, whose orientation can be high, there are few cases in which the chains can both undergo conformational changes and adopt more than one stable configuration (in order to allow switching response to an external field). As a consequence, only a few polymers are electromechanically active. Among polymers, only poly(vinylidene fluoride) (PVDF) and its trifluoroethylene copolymer p(VDF-TrFE) have had any commercial impact. The copolymers perform better than the homopolymers because they spontaneously crystallize into the all-trans xcex2-crystalline phase, and their Curie temperatures are below their melting points. These copolymers can have both piezoelectric and electrostrictive properties.
PVDF piezoelectric films have been commercially available since 1980, and are utilized in audio devices (microphones, high frequency speakers, and ultrasonic transducers), gauges (accelerometers, strain gauges, and load cells), and actuators (electronic fans and light shutters). However, the piezoelectric effect is more than an order of magnitude smaller than PZT, the most commonly used ceramic. For sonic transducer applications, PVDF-based materials can compete commercially with ceramics only as sound receivers, not as sound projectors.
The electrostrictive properties of p(VDF-TrFE) can be enhanced by forming crosslinked networks of the copolymer. Crosslinking reduces the size of the all-trans crystallites, making them more mobile and more able to react to an applied electric field. Irradiating the copolymer with high-energy electrons (Lovinger, Macromolecules 18, 910 (1985)) or gamma rays (Wang, Ferroelectrics 41, 213 (1982)) are known ways to form crosslinks. Zhang et al., Science 280, 2010 (1998) disclosed measurements of the electrostrictive properties of electron irradiated p(VDF-TrFE) films. The irradiated film exhibited strains as high as 4% in the thickness direction under an electric field of 150 MV/m. A disadvantage of irradiation is that it causes chain scission, isomerization, and bond rearrangement in addition to crosslinking. These side reactions create free radicals that can degrade the physical properties of the networked polymer. The degradation can continue for a period of years. Another drawback of irradiation is the inherent nonuniform energy distribution leading to nonuniform crosslink density.
Buckley et al., Appl. Phys. Lett. 78, 622 (2001) disclosed a chemical process to crosslink a cast film of a solution of p(VDF-TrFE) and a peroxide. The peroxide crosslinked the copolymer to form a network. This chemical process allowed for a more uniform crosslink density. However, the crosslink density was much lower than in irradiated films. Further, the chemically crosslinked network showed the same degree of chain scission as the irradiated films.
Logothetis, Prog. Polym. Sci. 14, 251 (1989) summarized chemical processes to crosslink vinylidene fluoride and its copolymers with incorporated cure site monomers. The cure site monomer contained either bromine or iodine. The copolymer was reacted with a peroxide and a radical trap. Crosslinking occurred only at the cure site monomers, and the reaction would not proceed in the absence of a cure site monomer. This process has the disadvantage that non-crosslinked cure site monomers can disrupt the crystallinity and degrade the electromechanical properties.
A process is needed to crosslink p(VDF-TrFE) with a crosslink density high enough to improve the electrostrictive properties while minimizing side reactions that degrade physical properties. The process should be a chemical process to provide uniform crosslink density. Further, the process should not require the use of extra cure site monomers. The process could be used to make an electrostrictive film with less weight than piezo-ceramics, but with improved toughness and processability.
It is an object of the invention to provide chemical processes to crosslink poly(vinylidene fluoride-co-trifluoroethylene) while minimizing degradative side reactions.
It is a further object of the invention to provide poly(vinylidene fluoride-co-trifluoroethylene) networked polymers with high electrostrictive coefficients, capable of higher strains in lower electric fields than the prior art compositions.
It is a further object of the invention to provide electrostrictive films comprising poly(vinylidene fluoride-co-trifluoroethylene) networked polymers.
These and other objects of the invention can be accomplished by a process for making a poly(vinylidene fluoride-co-trifluoroethylene) networked polymer comprising the steps of: providing a poly(vinylidene fluoride-co-trifluoroethylene) copolymer; mixing the copolymer with a peroxide and a coagent to form a curing mixture; and processing the curing mixture such that the peroxide, in combination with the coagent, crosslinks the copolymer to form the networked polymer.
The invention further comprises poly(vinylidene fluoride-co-trifluoroethylene networked) polymers made by the above process.
The invention further comprises electrostrictive films comprising the above networked polymer.